The problem is that Microsoft's OEM contracts mean that every PC sold by companies like HP, Dell, Lenovo and others all have a Windows license fee included in their price, whether or not the end-user chooses to run Windows. At one of my former employers we considered buying HP computers with Linux because we already had a license agreement with Microsoft that allowed us to install Windows XP on any machine we wanted so we were paying for the license twice by buying machines with Windows installed. The problem was the PC with Linux was more expensive than the one with Windows because you were paying for two licenses - Windows and Linux.
So each "PC" sold with OS X would be more expensive than with Windows because you have to pay the Windows license fee to Microsoft and then add on the OS X license fee to Apple. Some people might pay for that, but some will not. So that will incentivize Apple to add serial numbers and software activation and tying it all to a specific piece of hardware to curtail piracy, annoying all the people - especially Mac purchasers - who are paying for the product.
Also, by licensing OS X, Apple dilutes the user experience in terms of support. Because they currently control the hardware and the software, when you have a problem with your Mac, you go to one support source - Apple. They are forced to take ownership of the problem. In the case of an OEM, they have a financial incentive to pass you on to Apple who has one to pass you back to the OEM. So you can end up with Apple and the OEM tossing you back and forth like a shuttlecock. Also, will you be able to take an OEM product into an Apple Store for service and support - even if it's clearly an OS issue? If you can, Apple would need to spend a good bit more training Geniuses and those Geniuses might spend more time on each issue, meaning either more Geniuses are needed at the Bar or customers wait longer for a reservation.
And then there comes the hardware support. As annoying as it can be for a 68x00 or PowerPC Mac owner to be forced to replace your machine when Apple stops supporting it, by cutting the "support tail" after a certain period of time, you clean out the cruft in your OS. Windows has to support hardware going back 15 years and that is the source of a not insignificant number of performance and stability issues.
Now Apple could get around this by forcing OEMs to build to a "Reference Design" and selection of hardware so Apple's programmers do not need to write hundreds of drivers each for mice, sound cards, video cards, etc. But those who are hoping to slap OS X on any OEM machine will be in for disappointment.